A substrate for printed circuit boards, the substrate including a metal layer on a surface of an insulating base film has been widely used, the substrate being to be formed into a flexible printed circuit board by etching the metal layer to form a conductive pattern.
In recent years, trends toward miniaturization and higher performance of electronic devices require higher-density printed circuit boards. As a substrate for a printed circuit board that satisfies the demand for a higher density, a substrate for a printed circuit board has been require to include a conductive layer with a smaller thickness.
Such a substrate for a printed circuit board is also required to have a high peel strength between the base film and the metal layer so as not to allow the metal layer to be peeled from the base film when a flexible printed circuit board is subjected to a bending stress.
To deal with these demands, a substrate for a printed circuit board, the substrate including a thin copper layer laminated on a heat-resistant insulating base film without an adhesive layer provided therebetween has been reported (see Japanese Patent No. 3570802). In the substrate for a printed circuit board described in the patent publication, the thin copper layer is formed by a sputtering method on both surfaces of the heat-resistant insulating base film, and then a thick copper layer is formed thereon by an electrolytic plating method.